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World's largest switchboard for climate monitoring

Envisat ready for launch

26 February 2002

ESA Info 3-2002. Europe's showpiece in climate monitoring is called Envisat. Fully equipped, the largest, most complex and most powerful Earth observation satellite of the European Space Agency (ESA) is 25 metres high, ten metres wide and weighs over eight tons. It is scheduled for launch in the night of 28 February/1st March on an Ariane 5 launcher.

Europe's flying behemoth is on the trail of climate change. It will deliver data about global warming, ozone depletion and climate change for at least five years. The information is absolutely necessary and long overdue as the basis for political decisions about climate change.

Until now only a privileged few men and women have been able to see the Earth from outer space and to recognize how tiny and fragile it is. "I hope," said Alexei Leonow, the first cosmonaut to step out of his space vehicle, "that people understand that and protect our blue planet as their place of birth, as their common homeland and as the ancestral home where their children and grandchildren will live after them."

Envisat will fulfill this desire. With its ten instrument systems it is equipped with the best eyes possible and offers everything that scientists could wish for in the service of the environmentally endangered planet. The unique flying environment station follows in the footsteps of the successful European Earth remote sensing satellites ERS 1 (1991) and ERS 2 (1995).

Climate protection is a challenge for the entire society. This is why ESA did not hold back with the budget. The total costs for the Envisat Programme were €2.3 billion, distributed over 15 years. Included in this sum is development and construction of the instruments as well as the satellites, the launch on board Ariane 5 and the operational costs of the satellite for five years. Each citizen of the 15 ESA Member States thereby invested 7 Euros in the environment. Put another way, Envisat will cost each European citizen about one cup of coffee per year. For his money, for at least five years, the citizen receives precise information about changes in the environment including global warming, ozone depletion and climate change. This information is absolutely necessary and long overdue as the basis for political decisions. The gas envelope of the Earth is not determined by political boundaries. The atmosphere is global and the circulation is planetary. Letting down the environmental guard is not possible in Europe or anywhere else.

The sick Earth atmosphere

The effect of industrial activity in our atmosphere

The symptoms are unambiguous: the Earth's atmosphere is sick. How sick is being debated by the scientists. The threat to the environment appears in a fleeting disguise. It is made of gases which are streaming from the chimneys and smokestacks of our homes and factories, which are created by the combustion of wood, coal, oil and gas as well as slash and burn agriculture, the emissions from automobiles or airplanes, the flatulence of cows, and what spray cans, foaming agents, solvents, coolants and cleaning fluids release in the air. Among other such gases, are carbon monoxide, carbon dioxide, methane, ozone, nitrogen and CFC's but also water vapour. Although they make up on a small percentage of the total amount of trace gases, they could none the less become the instigators of global climate change. As is so often the case, it depends on the dose.

For several decades a steady increase in the atmosphere of climate-active gases has been registered. What is disturbing is that the processes are occurring by degrees. All substances released into the atmosphere have an effect, somewhere, somehow. The when, where and with which intensity and consequences is very difficult to understand because of the complexity of the climate processes and because the feed-back mechanisms are very difficult to predict. Everything is connected with everything else. "When I began to investigate the atmosphere, I concentrated exclusively on four chemical reactions which at the time were considered to be decisive", reports Paul Crutzen, who received the Nobel Prize for chemistry in 1995 for his investigation of the cause of the ozone hole. "Today we have to work with hundreds of reactions in our chemical models if we wish to understand something."

One cannot say with certainty in all cases whether certain substances are neutral or harmful. The best example is CFCs. They have enjoyed a remarkable career and metamorphosis. Discovered 70 years ago, they were long considered to be absolutely harmless, neutral and to a great extent environmentally benign because they are colourless and odourless and neither poisonous, corrosive, nor combustible. They were correspondingly in wide use - as a propellant in spray cans, coolant in refrigerators and air conditioners, as a foaming agent for plastic manufacture, as a sterilizing agent for medical implements, as solvent and cleaning fluid for cloth and leather, and as a cleaning agent for integrated circuits for the micro-electronics industry.

Environmentally benign CFCs were then declared ozone killers. In the meantime, that they destroy the ozone layer and cause the infamous ozone hole over the Antarctic has become common knowledge. The perpetrator and the victim can be widely separated from each other in time and place. Therefore the example makes clear how very important it is to have an overview of the Earth's atmosphere system in order to understand climate processes.

Science has its own way of acting. The mutual interests of theoreticians who develop the models of the atmosphere and the technicians who carry out measurements and build instruments proved to be extremely useful. The two climate celebrities, the long-time director of the Max-Planck-Institute for Chemistry at the University of Mainz, the Dutchman Paul Crutzen, and the Briton John Burrows, specialist in chemistry of the atmosphere at the Institute for Environmental Physics and Remote Sensing at the University of Bremen, personify a knowledge enriching symbiosis of this kind. They enjoy a long friendship with each other. In the mid 1980s, as director of an international group of scientists, John Burrows suggested the ozone instruments GOME and SCIAMACHY for the ERS satellite as well as Envisat and looked after these projects. Burrows has this to say about Crutzen: "He was an important supporter of the ESA-environmental satellite project Envisat. He is however primarily a modeler while I am an experimenter. That is how we continually supplement each other so well."

Ozone - the life-giving poison

Record Ozone Hole Extension during 2000

One of the most important greenhouse gases is ozone, a gas with a Janus face. We encounter it on the surface of the Earth and up to an altitude of 110 km. Approximately 10% is found in the troposphere, 90% in the stratosphere and the percentage in the mesosphere is insignificant. The greatest concentration of ozone is in the stratosphere from 18 to 30 km over the equator and 12 to 25 km over the upper latitudes.

Billions of years ago stratospheric ozone made possible life on this planet. It protects us today from the harmful effects of the ultraviolet radiation of the sun. We could not survive without the ozone in the atmosphere. Ozone in the upper troposphere also contributes somewhat as a filter against UV radiation. But ozone at ground level is a health problem. It is a main ingredient of smog. High ozone concentrations in the lower troposphere are poison for human, animal and plant life. "On the other hand we need ozone in the troposphere", explains Paul Crutzen, his voice rising, "because it is responsible for the formation of hydroxide radicals (OH). They ensure that almost all substances which make their way into the atmosphere become oxidized and removed from the atmosphere. Hydroxide radicals in combination with the UV radiation constitute the 'universal cleansing agent' of the atmosphere."

Holes in the protective shield

ERS-2/GOME map of ozone thinning over Europe

The problem of global warming is frequently confused with the ozone hole. There is a connection because ozone contributes to global warming and there are also mutual correlations. However they are two completely separate issues. In 1985 British researchers discovered the notorious ozone hole over Antarctica. It begins every year in spring in the southern hemisphere, which corresponds to our autumn. It only became clear years later that the dramatic ozone depletion was mainly due to chlorine radicals in the atmosphere and that the entire process depends on ice particles. A decisive part of the explanation of this riddle was provided by the Max-Planck-Institute for Chemistry in Mainz.

It had been going on already in the 1970s but was first identified in 1985. Every year the same process began over the South Pole: in the Antarctic spring, i.e. our autumn, the ozone layer in the stratosphere collapsed. The development was dramatic. From year to year the surface of the ozone hole grew larger. It is presently about 25 million square kilometers. At the same time the duration increased. The rapid thinning of the ozone starts at the end of September and lasts until December. Similar phenomena are increasing in the northern Hemisphere. Of course, according to Crutzen, "that is connected with certain meteorological phenomena and the intensities are highly variable." The ozone hole over the northern polar region in spring also occurs in the stratosphere at about 12 to 22 km altitude. This is also clearly caused by CFCs, that is, it has a man-made origin. Also in mid-latitudes, for instance over Berlin, already in March, at an altitude of 20 km, so-called polar stratospheric clouds have been detected. These are an aggressive mixture of ice and water particles, sulphur and potassium nitrate acid in which chlorine compounds are converted into ozone-killing chlorine radicals. The result is a region of reduced ozone which is often described as an ozone 'hole'.

The actual threat from increasing ozone depletion and holes in the atmosphere is the damage done to the UV screen. The ozone layer works like a pair of oversized sun glasses. Behind them, living creatures are spared the greater part of harmful UV radiation. The reduction of the ozone concentration in the stratosphere thereby leads to a weakening of this screen effect. The consequences are damage to the eyes, skin cancer and a weakening of the immune system. However UV radiation is also governed by strong natural variations. How much of the UV radiation is due to 'natural' causes and how much is due to ozone depletion is still unclear. Empirical investigations in northern and middle Europe show a UV rise per year of 0.5% in the last 10 years. During the time of the northern ozone maximum, i.e. from January until March, the UV radiation can even rise 30% in a short period of time. This is of particular concern for ski vacationers. Apropos vacations: in the future at beaches and mountain regions there will be a special UV radiation weather report, a kind of 'by-product' of Envisat's global, precise ozone measurement.

A new cycle of destruction discovered

Low-ozone event over Northern Hemisphere on 31 January 2002

A new ozone phenomenon at the North Pole in the lowest layer of the atmosphere, the troposphere, has been found. Several times during the arctic spring ozone disappears completely over an area of several thousand square kilometers. Paul Crutzen explains the situation: "The process happens very quickly. In just a few hours all the ozone can be destroyed. However in contrast to ozone depletion in the stratosphere this ozone hole has no negative effects. To the contrary: ground level ozone is unhealthy in higher concentrations; we notice this especially with smog at the height of summer. The bewildering observation was fascinating and at the same time unsettling. It shows that the chemistry of the atmosphere still has surprises in store and that science by no means understands all of the processes which occur in the atmosphere."

Measurements on Spit Bergen as well as in Canada showed that the ozone hole in the troposphere as well as its counterpart in the stratosphere have something in common: in each case some substance acts as a catalyst which reduces ozone to a normal diatomic oxygen molecule, but which is itself not used up. Therefore the smallest amounts of such a substance can have an enormously destructive effect. The catalyst in the stratosphere is chlorine and in the troposphere it is bromine. The GOME-detector of the European Earth reconnaissance satellite ERS-2 has provided the proof for this. One particle of bromine, bromine oxide (BrO) in 100 billion molecules of air, is enough to start the aggressive destruction of ozone.

If bromine is so aggressive, the question arises, why can it remain in the troposphere so long? After all, there are countless floating particles in this region, so-called aerosol particles, which capture this kind of substance and in this way, take it out of circulation. "More precise investigations show", as Crutzen explains the next bewildering realization, "that the aerosol particles in fact do exactly the opposite. In their interior chemical reactions take place which recycle inactive forms of bromine into reactive forms. One part of the aerosol consists of salt water droplets which contain bromine and which support the catalytic process even more."

Where does the bromine come from though? And why does the ozone depletion only take place in spring? Seawater is seen to be the main source of destructive bromine. One assumes that salt aerosols "form deposits during the arctic night at the borders of the pack ice and gradually accumulate there. As soon as the sun rises in the spring it activates the bromine in these deposits. If it is then carried away with fresh sea salt over the pack ice, it triggers the complete destruction of the ozone."

The experimenter John Burrows elaborates: "German researchers from Bremen and Heidelberg were the first to discover a troposphere ozone hole over the Antarctic. We are tracking the clouds with GOME and with the SCIAMACHY-Sensor on Envisat and hope to be able to clarify the many open questions about the newly discovered cycle of destruction." The Earth's atmosphere resembles a Swiss cheese. There are many ozone holes in the stratosphere as well as in the troposphere. In the stratosphere chlorine acts as the aggressive catalyst. The ozone holes over the Antarctic, the arctic as well as the middle latitudes are clearly man-made. They are a consequence of the anthropogenic introduction of CFCs. In contrast, in the troposphere bromine is acting as the catalyst. So far two ozone holes over the Arctic (January until March) and one over the Antarctic (autumn) have been identified. In these cases it is a natural phenomenon caused by the destructive effect of bromine which originates in sea water. There are still many unsolved matters in the climate who-dunnit. For Sherlock Holmes and Dr Watson the detail work comes first. "We are currently investigating with the computer", says Paul Crutzen, "Whether reactive halogens - chlorine, bromine and iodine - can also play a role in ozone chemistry in other regions and seasons."

Letting down the guard or apocalypse?

The Montreal Protocol was the first step to limit further damage to the ozone layer

Neither nor. The Montreal Protocol to Protect the Ozone Layer in 1987 was the first decisive step to limit further damage to the ozone layer in the stratosphere. There were subsequent steps and Europe played a leading role. But there are still too many loop holes for state sanctioned environmental transgressors in the industrial nations themselves. Ecological inaction has never hurt a politician, as long as regional or national planning only extends to the next election. Global environmental protection on the basis of secure scientific data, like those Envisat will provide, is therefore one of the most difficult political exercises. At the same time we have with Envisat, according to John Burrows, "an effective instrument for monitoring the Montreal and Kyoto treaties..." Furthermore: "Presumably we will be able to distinguish the anthropogenic and natural components of the most important substances in the atmosphere."

How long will we have to live with the ozone hole? For the realistic optimists, as Paul Crutzen likes to say, the measures are starting to show an effect. "In the best-case scenario the ozone hole will disappear in 40 years. Additional problems could arise, however. We have noticed that the lower stratosphere is getting cooler." Crutzen is alluding to the suspicion that the greenhouse effect in the troposphere could have disastrous consequences for the processes in the stratosphere. Measurements show that the water vapour content of the troposphere as well as the stratosphere is increasing. The extent of the latter is still unclear. Because of the cooling off in the lower stratosphere which has also been detected, formation of ice particles could increase, which could in turn activate the chlorine radicals. In the northern hemisphere and in the middle latitudes this could enable PSC clouds to form, which need relatively little chlorine to continue the ozone depletion. In other words, it could also take much longer, especially since some of the gases have a very long lifetime of up to 110 years. The situation with UV radiation in middle Europe is also unclear. Even the climate experts are poking about in the dark. It will supposedly be at least 40 to 50 years before the 'normal values' of the 1960s of the last century are reached again. John Burrows also expresses himself cautiously "For the moment one can't say anything about trends. For that we need a run of measurements. We're waiting for Envisat."

The greenhouse effect

Greenhouse gases' effect on global warming

Without the natural greenhouse effect it is certain that we would all be lost. The global average temperature would drop precipitously 33 degrees from its current 15° to -18°C. The Earth would congeal into an ice planet where life would cease to exist. Without the greenhouse effect life on our planet would not be possible at all. The principle is simple: trace gases of the Earth's envelope of air act like the pane of glass in a greenhouse. They let through the energy of visible sunlight but hold back the emission of the thermal radiation. As a consequence the air is heated and the temperature rises in the atmosphere. Water vapour alone accounts for a rise of 20.6ºC. Carbon dioxide accounts for 7.2ºC and the rest is taken up by atmospheric trace gases like ozone (2.4ºC), nitrogen dioxide (1.4ºC) and methane (0.8ºC).

John Burrows believes that "approximately 95% of the warming of the last 100 to 150 years was caused by human activity." In their basic assessment the experts are in agreement that it will very likely get warmer. Most of our contemporaries will be happy about that. But the human-induced greenhouse effect could have far-reaching consequences, like catastrophic droughts, desertification of agricultural regions, torrential rains and disastrous flooding of coastal areas, and the shifting of climate zones. The middle and northern latitudes could become substantially warmer and large areas of the permafrost would thaw. The world's sea levels, which have risen 10 to 20 cm in the last 100 years, could rise another 30 to 140 cm because of melting in glacial regions. This would have disastrous consequences for coastal regions, like Holland, and for river delta areas like the Ganges-Brahmaputra. Other stretches of land would be in danger of flooding.

However a precise prediction about the effects of the increasing greenhouse effect is not possible at present. Glaciers are sensitive early warning indicators but until now no clear conclusions can be drawn. A similar situation exists with the sea levels. In addition the oceans conceal and delay the effect in that they capture about half of the annual CO2 produced.

Climate changes proceed furtively, overlapped with natural variations. Until the meaningful 'signal' has been isolated from the general background climate 'noise', it could conceivably be too late. The "Core of the problem is an anthropogenically induced climate change which is occurring too gradually to be discovered before it is very advanced and which can be so far advanced that at the time it is discovered, it is too late." In order to confirm or disprove this effect Burrows says, "We need global models and global measurements over the next 20, 30 or 40 years. Then we will know which influence is playing which role."

How we guard against the anthropogenic greenhouse effect is a matter of extremely diverse opinions. A really efficient, rational use of energy must be assigned top priority. Energy efficient, high technology must be developed in order to broadly lower energy consumption. Technologies which do not release CO2 are required to replace fossil fuels. Worldwide reforestation is needed to remove more CO2 from the air. And in general, technologies which produce little or no byproducts or operate with closed cycles which do not burden the environment must be created.

Envisat, the flying climate station

The Global Ozone Monitoring Experiment–2 (GOME-2)

For four decades, weather, Earth reconnaissance and environmental satellites have been available for large-scale synoptic observation. In 1991 ESA began a very successful Earth reconnaissance programme with ERS-1, followed in 1995 with the world's best ozone lookout, ERS-2. With the Global Ozone Monitoring Experiment or GOME for the first time a global ozone map was assembled every three days which in rapid motion playback impressively visualizes the dramatic extent of the ozone hole.

Envisat is based on the experience of ERS and is in many categories a more completive satellite for 3-dimensional monitoring of the environment. The "most gigantic mission to planet Earth" is a well-deserved superlative. With ENVISAT, ESA has created a behemoth with a system as complex as the manifold processes of the environmental itself, regularly observing all important subsidiary processes in the atmosphere, the polar ice region, the oceans and the land. The exceptional comparability of the data constitutes at the same time the decisive prerequisite for the recognition of process dynamics on the Earth. "GOME was pretty successful", remembers John Burrows, "but with ENVISAT we can play worldwide in the big leagues. GOME was already advanced with respect to the American system but ENVISAT represents the absolute top performance for environmental measurements. We Europeans can depend on our own, independent set of data."

Three of the ten instruments are for climate research: GOMOS, MIPAS and SCIAMACHY. "With all three instruments one obtains the most exact data about ozone distribution and dozens of other trace gases which affect climate, from ground level to an altitude of 150 km. Taken together the measurements provide a complex picture of the chemistry of the atmosphere which helps us to fine tune the models", explains John Burrows.

Especially SCIAMACHY, Burrows continues, "will advance us in the research of air quality. This is a very topical and at the same time urgent theme. The emissions in the cities lead to ozone formation in an exhaust trail which affects the surrounding area on a large scale. This is how, when there is good weather like here in Bremen, the ozone health limits can easily be exceeded. SCIAMACHY will give us relevant data about the transport of these pollutants in the air so that we can trace their movement. That is a trans-continental problem. Part of our air pollution comes from the United States, we transport our pollution to Asia and the Asians pollute the United States..."

That is how planetary circulation works. But this example underscores how compellingly necessary it is to have a broad international collaboration in science, politics and application of the data. The ENVISAT-data are at the disposal of all scientists. Seven hundred international application projects are already running and the participants can hardly wait to begin the operational phase.

A change in climate is necessary

SCIAMACHY optical assembly

To summarize: prognoses about the future of the world climate are extremely complex. It will not suffice to identify single parameters and then to linearly extrapolate them. Models are necessary which with the new measurement data can be continually compared and modified. The great uncertainties are in coupled ocean-atmosphere models as well as the feedback mechanisms of individual parameters. Clouds are a major problem. Will they strengthen or weaken the expected greenhouse effect? Because of such uncertainties depending on the model, temperature rises fluctuate from 1.4 to 5.8ºC with the same starting temperature. "What we need in environmental research ", warns John Burrows "is long term and continuous flow of, and evaluation of, data. Therefore I am concerned that there is no successor for Envisat planned."

Burrow's concern, a mélange of warning and recommendation, is based on undeniable facts. The infamous ozone hole was discovered more by accident. Burrows and Crutzen should know: it could have been much worse. The Paul Crutzen reminisces in connection with the discovery of the ozone hole about how very necessary these continuous measurements are: "Since before 1974 no one was concerned about the effects of chlorine and bromine on the atmosphere, I can only say that we were lucky. This shows that we should be on our toes about the possible effects of the introduction of new products in the environment. A permanent surveillance of the composition of the stratosphere should enjoy high priority for many years to come". A lot of overtime is in store for our climate detectives in Mainz and Bremen. And not just in Mainz and Bremen.

Note to the Editors: all pictures relating to Envisat are available in low and high resolution under http://www.esa.int. The present information note is part of a series of articles devoted to the Envisat programme and its applications.

AerosolsA collection of tiny airborne solid or liquid particles, with a typical size between 0.01 and 10 microns (1 micron is one millionth of a metre). They stay in the atmosphere for at least several hours. Aerosols may be of either natural or anthropogenic origin. They may influence climate in two ways: directly through scattering and absorbing radiation, and indirectly through acting as condensation nuclei to assist cloud formation or modifying the optical properties and lifetime of clouds.

AlbedoThe fraction of solar radiation reflected by a surface or object, often expressed as a percentage. Clouds and snow-covered surfaces have a high albedo; the albedo of soils ranges from high to low; vegetation and oceans have a low albedo.

AnthropogenicResulting from or produced by human beings.

AtmosphereThe gaseous envelope surrounding the Earth. The dry atmosphere consists almost entirely of nitrogen (78.1% by volume) and oxygen (20.9% by volume), together with other trace gases such as argon (0.93%), carbon dioxide (0.035%) and ozone. Amounts of water vapour are highly variable but typically 1% by volume. The atmosphere also contains clouds and aerosols.

BiomassThe total mass of living organisms in a given area or volume. It includes dead organic matter.

BiosphereThe part of the Earth system comprising all living organisms - in the atmosphere, land (terrestrial biosphere), or oceans (marine biosphere) - as well as dead organic matter.

Carbon dioxide (CO2)A naturally occurring gas, also a by-product of burning fossil fuels and biomass, as well as land use changes and other industrial processes. The principal anthropogenic greenhouse gas.

ClimateThe 'average weather' as measured over a long period of time - typically 30 years.

Climate ChangeA statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period of time, typically decades or longer.

Climate modelA numerical representation of the climate system which attempts to include the physical, chemical and biological properties of its components, their interactions and feedback processes, while accounting for all or some of its known properties. Such models are used not only to study and simulate the climate, but for monthly, seasonal and long-term predictions.

CryosphereThe component of the climate system consisting of all snow, ice and permafrost on or beneath the surface of the Earth.

EcosystemA system of interacting, living organisms together with their physical environment. They may range from the very small scale (e.g. a pond) to the entire Earth.

El NiñoA warm water current that periodically flows along the coast of Ecuador and Peru, disrupting the local climate and fishery. This oceanic event is associated with the Southern Oscillation - a fluctuation in the pattern of surface air pressure and circulation in the Indian and Pacific Oceans. It has climatic effects throughout the Pacific and in many other parts of the world. The opposite of an El Niño event is called La Niña.

Energy balanceA long term balance, averaged over the entire globe, between incoming solar radiation and outgoing radiation - reflected solar radiation and infrared radiation emitted by the climate system. Human induced or natural changes to these inputs and outputs will upset the balance and may lead to global warming or cooling.

Fossil fuelsFossil carbon deposits e.g. coal, oil, and gas that are burned to produce energy. Their combustion results in emissions of carbon dioxide.

Greenhouse effectThe entrapment of heat by various greenhouse gases in the atmosphere. This occurs through the absorption of infrared radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. The natural greenhouse effect can be enhanced by an increase in the concentration of greenhouse gases through human activity.

Greenhouse gasesThe gaseous constituents of the atmosphere, both natural and anthropogenic that absorb and emit infrared radiation (heat). The most important of these are water vapour, carbon dioxide, nitrous oxide, methane and ozone. There are also a number of greenhouse gases that are solely of human origin, e.g.halocarbons

HalocarbonsCompounds containing either bromine, chlorine or fluorine and carbon. These can act as powerful greenhouse gases in the atmosphere. Those containing chlorine and bromine are also involved in the depletion of the ozone layer.

Infra-red radiationLong wave radiation emitted by the Earth’s surface, atmosphere and clouds, which is generally recognised as heat.

OzoneThe triatomic form of oxygen (O3) which occurs in the troposphere and the stratosphere. Tropospheric ozone acts as a greenhouse gas. It is created both naturally and by photochemical reactions involving gases produced by human activities (“smog”). Stratospheric ozone is an important contributor to the Earth’s radiation balance. Its concentration is highest in the ozone layer.

SinkAny process, activity or mechanism which removes a greenhouse gas, an aerosol or their precursors from the atmosphere.

StratosphereThe region of the atmosphere above the troposphere, extending from about 10 km to about 50 km above the surface.

TroposphereThe lowest part of the atmosphere where clouds and weather occur. Its upper boundary (the tropopause) lies at an altitude of about 9 km near the poles, rising to 16 km in the tropics. In the troposphere, temperatures generally decrease with height.